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Digitale Medien

Grain-boundary compositions in YBa2Cu3O7−x from Auger electron spectroscopy of fracture surfaces (1988)

Kroeger, D. M. ; Choudhury, A. ; Brynestad, J. ; [weitere]

[S.l.] : American Institute of Physics (AIP)

Journal of Applied Physics 64 (1988), S. 331-335

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ISSN: 1089-7550

Quelle: AIP Digital Archive

Thema: Physik

Notizen: Comparison of Auger electron spectra obtained from intergranular and transgranular areas exposed on fracture surfaces of sintered YBa2Cu3O7−x specimens indicates that most grain-boundary surfaces are deficient in oxygen and rich in copper compared to the bulk. The thickness of this region of altered composition is estimated to be in the range of 15–50 A(ring). No evidence of segregation of impurities to grain boundaries was seen. The results suggest that the grain-boundary layer is nonsuperconducting and a likely contributor to the problem of low critical current densities in these materials. It is believed that carbon at grain boundaries which has been reported results from incomplete calcination in material which is slightly off stoichiometry.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.341432

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Digitale Medien

Preparation and grain boundary chemistry of dense polycrystalline YBa2Cu4O∼8 (1991)

Williams, R. K. ; Brynestad, J. ; Henson, T. J. ; [weitere]

[S.l.] : American Institute of Physics (AIP)

Journal of Applied Physics 69 (1991), S. 2426-2430

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ISSN: 1089-7550

Quelle: AIP Digital Archive

Thema: Physik

Notizen: Dense polycrystalline samples of the compound YBa2Cu4O∼8, Y124, were prepared by a three-step procedure. In the first step a mixture of Y2O3, CuO and BaO2 was reacted at 960 °C under an oxygen pressure of 55 bar. The compound did not sinter under these conditions but electrical resistivity and susceptibility measurements showed that the oxide mixture converted to Y124. In the second step densification was produced by hot isostatic pressing at 875 °C. This degraded the superconducting properties severely and a final high-temperature anneal in pressurized oxygen was required to restore optimum properties. This anneal consisted of a one hour hold at 1010 °C in 40-bar O2 followed by a 24-h anneal at 950 °C in 55-bar O2. After this treatment the samples were ∼90% dense, phase pure Y124. The maximum grain dimensions were about 10 μm, and Auger spectroscopy data were obtained on a fracture surface. These data showed that the grain boundary composition does not differ significantly from the bulk values.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.348677

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Digitale Medien

Changes in fracture surface composition and morphology with time in vacuum in YBa2Cu3O7−x (1988)

Kroeger, D. M. ; Brynestad, J. ; Padgett, R. A.

Woodbury, NY : American Institute of Physics (AIP)

Applied Physics Letters 52 (1988), S. 1266-1267

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ISSN: 1077-3118

Quelle: AIP Digital Archive

Thema: Physik

Notizen: Changes in fracture surface composition and morphology with time after fracture in a high vacuum environment are observed in the high transition temperature oxide superconductor YBa2 Cu3 O7−x . Morphological changes involving several grains and extending over distances of tens of microns are seen. Changes in fracture surface morphology are, at least in some cases, preceded by changes in surface composition as determined from Auger electron spectroscopy, and can occur at room temperature in the absence of electron beam excitation.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1063/1.99677

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Digitale Medien

Splicing of Messenger RNA Precursors (1986)

Padgett, R A ; Grabowski, P J ; Konarska, M M ; [weitere]

Palo Alto, Calif. : Annual Reviews

Annual Review of Biochemistry 55 (1986), S. 1119-1150

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ISSN: 0066-4154

Quelle: Annual Reviews Electronic Back Volume Collection 1932-2001ff

Thema: Chemie und Pharmazie , Biologie

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1146/annurev.bi.55.070186.005351

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Digitale Medien

Sulfidation behavior of Fe-25Cr-20Ni-1.5Al2O3 in simulated coal combustion/gasification environments (1991)

Srinivasan, V. ; Harris, L. A. ; Padgett, R. A. ; [weitere]

Springer

Oxidation of metals 36 (1991), S. 195-219

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ISSN: 1573-4889

Schlagwort(e): sulfidation ; Fe-25Cr-20Ni-1.5Al2O3 ; oxide dispersion-strengthened alloy ; mixed gas corrosion ; coal combustion/gasification

Quelle: Springer Online Journal Archives 1860-2000

Thema: Maschinenbau

Notizen: Abstract The effect of minor addition of α-Al2O3 dispersoids on the sulfidation behavior of Fe-25Cr-20Ni was investigated over a range of pO2, 1.13×10−20 to 1.18×10 ****−22 atm. at constant pS2=1.22×10−8 atm. Fe-25Cr-20Ni and Fe-25Cr-20Ni 1.5 Al2O3 with and without preformed oxide scales were exposed to bioxidant gas mixtures H2/H2O/H2S/Ar at 700° C. Both isothermal and cyclic exposures were included. Scales were characterized by a combination of several surface analytical tools. A remarkable improvement in sulfidation resistance is observed in Fe-25Cr-20Ni-1.5Al2O3 under the conditions investigated here. This is attributed to the ability of the alloy to form and maintain a predominantly Cr2O3 scale with reduced Fe-diffusion and content. Possible scientific reasons for such improvement are discussed. The base alloy, Fe-25Cr-20Ni, fails to develop and retain such a Cr2O3 scale and undergoes sulfidation within a few minutes of exposure. The scale breakdown process by sulfidation is explained qualitatively. Experimental evidence suggests that sulfur in the environment enhances Fe-diffusion and content in the scale.

Materialart: Digitale Medien

URL: http://dx.doi.org/10.1007/BF00662962

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